1. Field of the Invention
The present invention relates to a seal or packing assembly for sealing the annulus between an outer cylindrical surface of one member and an inner cylindrical surface of another member, with at least one member having a through bore for passing fluid through the member. More particularly, the present invention relates to a mechanically or hydraulically set packing assembly for a crossover seal assembly, a tubing hanger, a subsea wellhead, a surface wellhead or christmas tree, or similar downhole or surface equipment used in hydrocarbon recovery operations and having a flow path therein for receiving a tubular member to recover formation fluids.
2. Description of the Background
Various downhole and wellhead equipment having an internal flow path for transmitting formation fluids requires a packing assembly for reliably sealing the annulus between an outer cylindrical surface provided by the equipment and an inner cylindrical surface provided by a casing or tubing string positioned within the equipment. Such a packing assembly is used, for example, in a crossover seal assembly to seal the upper end of a casing with a casing head having a central bore defining an outer sealing surface, and thus provides a seal between the casing and the casing head.
A packing assembly may be used within a wellhead located at the surface, but also is frequently used in downhole equipment which may be spaced hundreds of meters below the surface where installation and repair are both difficult and expensive. Most prior art packing assemblies used in wellheads and downhole equipment have required that the cylindrical surfaces which will be in sealing engagement with the packing assembly be specially machined to obtain the desired surface finish and machine tolerances required for a reliable seal. Packing assemblies traditionally have used various elastomeric materials to form the seal with these machined surfaces, and improvements in elastomeric materials have increased the reliability of such sealing assemblies under some conditions. Exemplary sealing assemblies with rubber or elastomeric sealing components are depicted in U.S. Pat. Nos. 2,589,483 and 2,591,142.
Packing assemblies with elastomeric seals have at least two significant drawbacks which have limited their acceptance in the hydrocarbon recovery industry: (1) for the elastomeric seal to be effective, the metal surfaces which the seal engages typically must be specially machined, thereby reducing the versatility of field operations and increasing costs, and (2) such seals are susceptible to significant deterioration or total destruction at elevated temperatures caused by fires or other high temperature environments. Since such packing assemblies should reliably seal when subjected to fluid pressures in excess of five thousand PSI and since such packing assemblies are commonly critical in the event of a fire or other emergency situation, these limitations frequently lead the production operator to require a packing assembly which will form a metal-to-metal seal and thereby overcome these limitations.
One type of packing assembly with seals for metal-to-metal engagement with the inner and outer cylindrical surfaces is offered by WKM in Houston, Texas and is designated as a CANH seal. This type of packing assembly overcomes the undesirable features of a seal with elastomeric components, i.e., it does not require machined surfaces for reliable sealing engagement, and is not susceptible to immediate deterioration in a fire. Nevertheless, this sealing assembly has other drawbacks which have limited its acceptance in the hydrocarbon recovery industry. The packing assembly comprises separate upper and lower seals with mating surfaces. Each seal is individually placed into position for service, so that multiple "trips" are necessary to position the packing assembly in a downhole environment. Since two sets of packing assemblies each having upper and lower seal rings are frequently utilized, these multiple trips increase the installation costs and undesirable downtime. Moreover, each set of upper and lower seals preferably is provided with top and bottom protruding portions for forming substantially line contact sealing edges with the corresponding cylindrical surface. The design of the packing assembly does not result, however, in substantially equal radial pressure being applied to each of these sealing edges, thus reducing the reliability of the seal. The WKM crossover packing assembly is more fully described in U.S. Pat. No. 4,556,224.
The disadvantages of the prior art are overcome the present invention, and an improved packing assembly suitable for use with various downhole or wellhead equipment is hereinafter provided, along with an improved method for positioning the packing assembly in place and thereafter setting the packing assembly.